Formation Of Transient Species Research Articles

Iron insertion into coproporphyrin III-ferrochelatase complex: evidence for an intermediate distorted catalytic species.

Understanding the reaction mechanism of enzymes at the molecular level is generally a difficult task, since many parameters affect the turnover. Often, due to high reactivity and the formation of transient species or intermediates, detailed information on enzymatic catalysis is obtained by means of model substrates. Whenever possible, it is essential to confirm a reaction mechanism based on substrate analogues or model systems by using the physiological substrates. Here we disclose the ferrous iron incorporation mechanism, in solution and in crystallo, by the coproporphyrin III-coproporphyrin ferrochelatase complex from the firmicute, pathogen and antibiotic resistant, Listeria monocytogenes. Coproporphyrin ferrochelatase plays an important physiological role as the metalation represents the penultimate reaction step in the prokaryotic coproporphyrin-dependent heme biosynthetic pathway, yielding coproheme (ferric coproporphyrin III). By following the metal titration with resonance Raman spectroscopy and X-ray crystallography, we prove that upon metalation the saddling distortion becomes predominant both in the crystal and in solution. This is a consequence of the readjustment of hydrogen bond interactions of the propionates with the protein scaffold during the enzymatic catalysis. Once the propionates have established the interactions typical of the coproheme complex, the distortion slowly decreases, to reach the almost planar final product. This article is protected by copyright. All rights reserved.

Aqueous-phase chemistry and bactericidal effects from an air discharge plasma in contact with water: evidence for the formation of peroxynitrite through a pseudo-second-order post-discharge reaction of H2O2 and HNO2

The formation of transient species (OH·, NO2·, NO radicals) and long-lived chemical products (O3, H2O2, , ) produced by a gas discharge plasma at the gas–liquid interface and directly in the liquid was measured in dependence on the gas atmosphere (20% oxygen mixtures with nitrogen or with argon) and pH of plasma-treated water (controlled by buffers at pH 3.3, 6.9 or 10.1). The aqueous-phase chemistry and specific contributions of these species to the chemical and biocidal effects of air discharge plasma in water were evaluated using phenol as a chemical probe and bacteria Escherichia coli. The nitrated and nitrosylated products of phenol (4-nitrophenol, 2-nitrophenol, 4-nitrocatechol, 4-nitrosophenol) in addition to the hydroxylated products (catechol, hydroquinone, 1,4-benzoquinone, hydroxy-1,4-benzoquinone) evidenced formation of NO2·, NO· and OH· radicals and NO+ ions directly by the air plasma at the gas–liquid interface and through post-discharge processes in plasma-activated water (PAW) mediated by peroxynitrite (ONOOH). Kinetic study of post-discharge evolution of H2O2 and in PAW has demonstrated excellent fit with the pseudo-second-order reaction between H2O2 and . The third-order rate constant k = 1.1 × 103 M−2 s−1 for the reaction was determined in PAW at pH 3.3 with the rate of ONOOH formation in the range 10−8–10−9 M s−1. Peroxynitrite chemistry was shown to significantly participate in the antibacterial properties of PAW. Ozone presence in PAW was proved indirectly by pH-dependent degradation of phenol and detection of cis,cis-muconic acid, but contribution of ozone to the inactivation of bacteria by the air plasma was negligible.

Photolysis of 6-ethoxy-2,2,4-trimethyl-8-nitro-1,2-dihydroquinoline. Unusual dependence on the irradiation wavelength

The spectral and kinetic parameters of transient species generated in the irradiation of 6-ethoxy-2,2,4-trimethyl-8-nitro-1,2-dihydroquinoline were examined by stationary and pulse photolysis in the solvents: heptane, acetonitrile, methanol, and ethanol. Upon excitation of the long-wavelength absorption band (λex > 450 nm), a reversible photochemical reaction was revealed, and the spectral and kinetic parameters of three transient species observed in the photolysis were characterized (λmax = 390, 400, and 420 nm (acetonitrile), k = 97, 500, and 2000 s−1, respectively). The absorption spectra and the rate constants of the decay of transient species are almost independent of the medium polarity and the presence of oxygen in the system. The excited state generated during irradiation to the short-wavelength absorption band (290 < λex < 350 nm) is inactive in the photochemical reaction and deactivates without the formation of transient species. The mechanism of the reversible photochemical reaction is suggested, which involves the opening of the heterocycle N-C bond upon photoexcitation of the long-wavelength absorption band and the thermal back reaction.

Oxidations by the system ‘hydrogen peroxide–[Mn 2L 2O 3] 2+ (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)–oxalic acid’. Part 11. Degradation of dye Rhodamine 6G and oxygenation of cyclohexene

Two dinuclear manganese(IV) compounds, soluble [LMn(O) 3MnL](PF 6) 2 ( 1a) and insoluble [LMn(O) 3MnL] 2[SiW 12O 40] ( 1b) (L is 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) are efficient catalysts in oxidative decoloration of dye Rhodamine 6G and oxygenation of cyclohexene with hydrogen peroxide in acetonitrile or water in the presence of oxalic acid which is an obligatory co-catalyst. It has been concluded on the basis of the detailed kinetic study that both oxidation processes occur with the formation of transient species D (which exhibits the properties of a relatively weak radical and is able to abstract the hydrogen atom from C–H bonds of hydrocarbons) and E (which is responsible for the cyclohexene epoxidation). Species D is probably an oxygen-centered radical containing also manganese ions. Species E is apparently an oxoderivative of high-valent manganese. Catalyst 1a generates predominantly species E. When heterogenized catalyst 1b is used species D prevail. As a very intensive dye degradation occurs also in aqueous solution the studied systems can be used for decoloration of pollutants in waste streams.

Generation and reactivity of the radical cations of coniferyl alcohol and isoeugenol in solution

Nanosecond laser flash photolysis of coniferyl alcohol and isoeugenol in acetonitrile leads to the formation of transient species that are identified as the corresponding radical cations. These radical cations decay with rate constants of ca. 1 × 106 s–1 in dry acetonitrile. Both radical cations react rapidly with hydroxylic solvents like water and alcohols to give 4-vinylphenoxyl radicals, indicating that these reagents behave as bases rather than nucleophiles. In addition, anionic reagents (acetate, cyanide, and chloride) react rapidly with the radical cations with second-order rate constants that are close to diffusion controlled. The main products generated in the presence of the anionic reagents are again the 4-vinylphenoxyl radicals, suggesting that these reagents also behave as bases. The lifetime of the radical cations in acidic acetonitrile was found to increase dramatically due to a shift in the radical cation – vinyl phenoxyl radical acid–base equilibrium to the side of the radical cation. An estimate of the pKa of the radical cation in acetonitrile of 4.0 was obtained from the data.Key words: radical cations, laser flash photolysis, lignan, vinylphenols.

Flash photolysis and pulse radiolysis of the Co(sep)3+-X- (sep = sepulchrate; X = I, Br) systems in aqueous solution

The Co(sep)/sup 3 +/ complex (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)eicosane) in aqueous solution forms ion pairs with the I/sup -/ and Br/sup -/ anions, which exhibit a relatively intense charge-transfer absorption. In deoxygenated aqueous solution no net reaction is obtained upon continuous light excitation in the ion-pair charge-transfer bands, but formation of transient species is observed in flash photolysis experiments. For the Co(sep)/sup 3 +/-I/sup -/ system, I/sub 2//sup -/ ions are formed that decay in the 50-..mu..s time scale to give I/sub 3//sup -/. The latter species disappears in a time scale of seconds, leading the system back to the preexcitation conditions. For the Co(sep)/sup 3 +/-Br/sup -/ system, only formation of the Br/sub 2//sup -/ transient is observed, followed by regeneration of the preexcitation conditions in the 20-..mu..s time scale. In order to elucidate the kinetic aspects of the transient formation and disappearance, pulse radiolysis experiments on the Co(sep)/sup 3 +/-I/sup -/ and Co(sep)/sup 3 +/-Br/sup -/ systems have been carried out. The rate constants of the reactions of Co(sep)/sup 2 +/ with I/sub 2//sup -/, I/sub 3//sup -/, and Br/sub 2//sup -/ have been measured, and a complete picture of the redox processes that follow flash photolysismore » and pulse radiolysis excitations has been obtained. 5 figures.« less

E.S.R. studies of free radicals derived from reaction of OH with uracil and thymine.

Short-lived free radicals produced in N2O-saturated aqueous solutions of uracil and thymine have been studied using the in situ radiolysis steady-state e.s.r. method. Radical formed in alkaline aqueous solutions by OH addition to either positions C(5) or (6) were observed. Mechanisms for the formation of transient species were derived. The spin density distribution of the unpaired electron was calculated by means of the INDO method.